With high integration and large capacity of a Large Scale Integration (LSI), a circuit dimension required for a semiconductor element has become increasingly narrowed.
Recently, EUV (Extreme Ultraviolet) lithography and nanoimprint lithography (NIL) have attracted attention as technologies for forming fine patterns on a semiconductor wafer.
In the EUV lithography, a pattern formed in a mask is transferred onto a resist film formed on a wafer by an exposure apparatus by illuminating extreme ultraviolet light. On the other hand, in the nanoimprint lithography, the fine pattern is transferred on the resist film by pressuring a template having a fine pattern of a nanometer-scale fine structure to a resist film formed on a wafer. In both EUV lithography and the nanoimprint lithography, a pattern formed in the EUV mask and the template that is an original plate, is finer when compared with ArF lithography using an exposure apparatus by illuminating ArF light. Therefore, in an inspection process for a mask used for EUV lithography, and in an inspection process for a template used for nanoimprint lithography, high inspection accuracy is required to detect a defect.
In the mask inspection process, a pattern region of the mask is virtually divided into a plurality of strip-shaped stripe regions, and an optical image of a pattern is acquired by scanning inspection light on each stripe region in a longitudinal direction. The scanning by the inspection light is actually performed by a movement of a stage on which a mask is placed. Then, the existence or non-existence of a defect of a pattern in each stripe region is examined using the acquired optical image. The acquisition of the optical image is continuously performed. When one stripe region is scanned by the inspection light, an adjacent stripe region is then scanned. With this process repeated, the inspection is performed such that the entire pattern region is scanned. In addition, the same process is also applied to a case where the inspection target is a template. Specific examples of the inspection method are disclosed in, for example, JP2013-40873, JP2005-235777, and JP2009-192345.
One of the important items to consider in the defect determination in the inspection of the mask or the template is a misplacement of the pattern. In order to obtain a misplacement amount of the pattern, it is necessary to measure an accurate position of the pattern. In practice, this is obtained by measuring position coordinates of a stage on which the mask or the template is placed. For example, the misplacement amount of the pattern is obtained by obtaining position coordinates of the pattern from a measurement value of the position coordinates of the stage and a positional relationship between the stage and the mask, and obtaining a misplacement amount between the obtained position coordinates of the pattern and design coordinates of the pattern. In this case, the position coordinates of the stage is measured by a laser interferometer provided in an inspection apparatus.
The laser interferometer is an apparatus that measures the position coordinates of the stage by laser light, emitted from a laser head, incident to a stage mirror fixed to the stage, or by reflecting laser light emitted from a laser head from a stage mirror fixed to the stage. As described above, since the position coordinates on the mask can be known from the position coordinates of the stage measured by the laser interferometer, and the positional relationship between the stage and the mask, the position coordinates of the pattern formed in the mask are obtained from the above position coordinates.
Incidentally, in the laser interferometer, a wavelength of the laser light is a reference when a distance is measured. The wavelength of the laser light is changed according to a refractive index of a medium through which the laser light propagates. In the case of the laser interferometer used for measuring the position coordinates of the stage in the inspection apparatus, the medium through which the laser light propagates is air and the refractive index thereof is changed according to atmospheric pressure or temperature change. If the refractive index is changed, the wavelength of the laser light is changed and the measurement value is changed. Therefore, the position coordinates of the stage cannot be accurately obtained. As a result, the accurate position coordinates of the pattern formed in the mask or the template cannot be obtained.
Therefore, it may be considered that the atmospheric pressure or the temperature is monitored by an atmospheric pressure sensor or a temperature sensor, and a measurement error occurring due to a wavelength change caused by these changes is corrected. However, it is difficult to sufficiently reduce the measurement error through only such correction. This problem is particularly severe in an inspection target requiring high inspection accuracy, such as a mask in EUV lithography or a template in nanoimprint lithography. A solution to this problem is therefore urgently needed.
The present invention has been made in view of the above-mentioned problems. That is, an object of the present invention is to provide an inspection method that can perform inspection by accurately measuring position coordinates of an inspection target.
Other challenges and advantages of the present invention are apparent from the following description.